1. Field of the Invention
The present invention relates to a joint structure for a sandwiched structural body comprising a core layer whose opposite surfaces are covered with outer skin layers, and more particularly to a joint structure for joining a component to a sandwiched structural body used as a motor vehicle body, or joining sandwiched structural bodies to each other.
2. Description of Relevant Art
Composite structural bodies mainly made of synthetic resin or reinforced plastic have recently been proposed and used as motor vehicle bodies for the purposes of reducing the weight of motor vehicles and also reducing the number of motor vehicle components for greater productivity of motor vehicles.
Such composite structural bodies are generally in the form of a sandwiched structure body comprising a core layer whose opposite surfaces are covered with outer skin layers. The core layer may be made of foamed plastic (see, for example, Japanese Patent Publications Nos. 46-36329 and 55-43912, Japanese Laid-Open Utility Model Publication No. 63-30276, U.S. Pat. Nos. 4,559,274 and 4,573,707), or of a honeycomb structure (see, for example, Japanese Laid-Open Patent Publication No. 47-32521, Japanese Laid-Open Utility Model Publications Nos. 57-114774, 62-174935, and 62-174978). It has also been proposed to use, as a motor vehicle body, a sandwiched structural body with a hollow space in a core layer, as disclosed in Japanese Laid-Open Utility Model Publication No. 62-166136, for example.
Modern motor vehicles such as automobiles mostly have a monocoque body that combines a frame and a body together and which can bodily bear external forces applied thereto. Since the applied external forces are distributed over the entire monocoque body, it can withstand strong stresses. The monocoque body is advantageous in that the entire motor vehicle weight is reduced, and the motor vehicles can be produced at an increased production rate and with less cost. Therefore, it is preferable that motor vehicle bodies in the form of composite structural bodies of reinforced plastic or the like be also of a monocoque structure for reduced motor vehicle weight and increased productivity. Demands for composite structural bodies as monocoque motor vehicle bodies are growing.
However, there are known certain problems in connection with sandwiched composite structural bodies of synthetic resin or reinforced plastic. Specifically, a component joined to a core layer by screws or other fasteners may not be fastened strongly enough and may work loose. Sufficient bonding strength cannot be achieved either when a component is welded or bonded to an outer skin layer. Components in the form of sandwiched structural bodies cannot easily be joined by fasteners to each other because of the core layer between the outer skin layers of each sandwiched structural body. When the outer skin layers of such components are bonded together by an adhesive, no sufficient bonding strength is available between the components. If a monocoque motor vehicle body were assembled from a plurality of components in the form of sandwiched composite structural bodies, as with conventional steel sheets or the like, then the assembled monocoque body would not be sufficiently mechanically strong, durable, and shock-resistant. Such an approach would not be effective to increase productivity as the number of components would not be reduced.